Adds safe constructor and initialization for Heap

src/lib.rs

@@ -17,6 +17,7 @@ use core::alloc::GlobalAlloc;
 use core::alloc::Layout;
 #[cfg(feature = "alloc_ref")]
 use core::alloc::{AllocError, Allocator};
+use core::mem::MaybeUninit;
 #[cfg(feature = "use_spin")]
 use core::ops::Deref;
 use core::ptr::NonNull;
@@ -73,6 +74,32 @@ impl Heap {
         self.holes = HoleList::new(heap_bottom, heap_size);
     }
 
+    /// Initialize an empty heap with provided memory.
+    ///
+    /// The caller is responsible for procuring a region of raw memory that may be utilized by the
+    /// allocator. This might be done via any method such as (unsafely) taking a region from the
+    /// program's memory, from a mutable static, or by allocating and leaking such memory from
+    /// another allocator.
+    ///
+    /// The latter method may be especially useful if the underlying allocator does not perform
+    /// deallocation (e.g. a simple bump allocator). Then the overlaid linked-list-allocator can
+    /// provide memory reclamation.
+    ///
+    /// # Panics
+    ///
+    /// This method panics if the heap is already initialized.
+    pub fn init_from_slice(&mut self, mem: &'static mut [MaybeUninit<u8>]) {
+        assert!(self.bottom == 0, "The heap has already been initialized.");
+        let size = mem.len();
+        let address = mem.as_ptr() as usize;
+        // SAFETY: All initialization requires the bottom address to be valid, which implies it
+        // must not be 0. Initially the address is 0. The assertion above ensures that no
+        // initialization had been called before.
+        // The given address and size is valid according to the safety invariants of the mutable
+        // reference handed to us by the caller.
+        unsafe { self.init(address, size) }
+    }
+
     /// Creates a new heap with the given `bottom` and `size`. The bottom address must be valid
     /// and the memory in the `[heap_bottom, heap_bottom + heap_size)` range must not be used for
     /// anything else. This function is unsafe because it can cause undefined behavior if the
@@ -90,6 +117,18 @@ impl Heap {
         }
     }
 
+    /// Creates a new heap from a slice of raw memory.
+    ///
+    /// This has the same effect as [`init_from_slice`] on an empty heap, but it is combined into a
+    /// single operation that can not panic.
+    pub fn from_slice(mem: &'static mut [MaybeUninit<u8>]) -> Heap {
+        let size = mem.len();
+        let address = mem.as_ptr() as usize;
+        // SAFETY: The given address and size is valid according to the safety invariants of the
+        // mutable reference handed to us by the caller.
+        unsafe { Self::new(address, size) }
+    }
+
     /// Allocates a chunk of the given size with the given alignment. Returns a pointer to the
     /// beginning of that chunk if it was successful. Else it returns `None`.
     /// This function scans the list of free memory blocks and uses the first block that is big

src/test.rs

@@ -1,25 +1,28 @@
 use super::*;
 use core::alloc::Layout;
-use std::mem::{align_of, size_of};
+use std::mem::{align_of, size_of, MaybeUninit};
 use std::prelude::v1::*;
 
 fn new_heap() -> Heap {
     const HEAP_SIZE: usize = 1000;
-    let heap_space = Box::into_raw(Box::new([0u8; HEAP_SIZE]));
+    let heap_space = Box::leak(Box::new([MaybeUninit::uninit(); HEAP_SIZE]));
+    let assumed_location = heap_space.as_ptr() as usize;
 
-    let heap = unsafe { Heap::new(heap_space as usize, HEAP_SIZE) };
-    assert!(heap.bottom == heap_space as usize);
+    let heap = Heap::from_slice(heap_space);
+    assert!(heap.bottom == assumed_location);
     assert!(heap.size == HEAP_SIZE);
     heap
 }
 
 fn new_max_heap() -> Heap {
     const HEAP_SIZE: usize = 1024;
     const HEAP_SIZE_MAX: usize = 2048;
-    let heap_space = Box::into_raw(Box::new([0u8; HEAP_SIZE_MAX]));
+    let heap_space = Box::leak(Box::new([MaybeUninit::<u8>::uninit(); HEAP_SIZE_MAX]));
+    let start_ptr = heap_space.as_ptr() as usize;
 
-    let heap = unsafe { Heap::new(heap_space as usize, HEAP_SIZE) };
-    assert!(heap.bottom == heap_space as usize);
+    // Unsafe so that we have provenance over the whole allocation.
+    let heap = unsafe { Heap::new(start_ptr, HEAP_SIZE) };
+    assert!(heap.bottom == start_ptr);
     assert!(heap.size == HEAP_SIZE);
     heap
 }